Fiber filled plastic compounds suitable for injection molding have become widely used. The fibers impart many valuable characteristics to the injection molded articles, foremost of which are high dimensional stability, high modulus of elasticity, high resistance to distortion by heat, high tensile strength, unusually high flexural modulus and low shrinkage during curing. Glass-reinforced thermoplastic injection molding compounds and injection molding processes employing them are described in Bradt, U.S. Pat. No. 2,877,501. The technology of the Bradt patent has subsequently been extended. In addition to the styrene resins, styrene-acrylonitrile copolymer resins and styrene-butadiene copolymer resins described therein, numerous other injection-moldable thermoplastic resins, such as polycarbonate resins, acrylonitrile-butadiene-styrene terpolymer resins, poly (ethylene terephthalate) resins, polysulfone resins, polyphenylene ether resin, nylon resins, and the like, are effectively reinforced by glass fibers. Moreover, instead of glass fibers, subsequently developed commercial products are reinforced with filaments of carbon fibers, graphite fibers, aramid fibers, stainless steel filaments and others, as well as mixtures of any of the foregoing, many such products stemming directly from the technology disclosed in the above-mentioned U.S. Pat. No. 2,877,501. Such technology involves providing elongated granules, each of the granules containing a bundle of elongated reinforcing filaments extending generally parallel to each other longitudinally of the granule and a thermoplastic molding composition surrounding and permeating the bundle. In the process of injection molding, such granules are forced into a mold, wherein the filaments will be dispersed and produce molded articles with improved properties in comparison with the molded thermoplastic alone.
The above-mentioned U.S. Pat. No. 2,877,501, discloses pellets comprising 15-60 wt. % glass in thermoplastic resin, e.g., polystyrene. This corresponds to 8.1%-42.9% of filaments by volume and correspondingly 91.9-57.1% by volume of resin. Current processes for making such prior art filament-filled granules require a compounding/pelletizing step, in which the thermoplastic material is mixed with filaments, usually chopped bundles of filaments, and usually in an extruder, then the extrudate is chopped into molding granules. Such equipment is not readily available to the molder, and a number of specialty compounders have established businesses in which fibers from one source, and thermoplastics from another source are formulated into granules in drums or truckloads for sale to molders. It would be desirable to by-pass such compounders and permit molders to feed mixtures of thermoplastics and fibers directly into the molding press hopper achieving fiber dispersion by shear forces at the screw, nozzle, check valve, runners, gates, etc., in the injection molding machine. It would also be desirable to use, in comparison with the prior art, much less resin in the pellets, e.g., 2.5-32.5% by volume (instead of 57.1-91.9%) and much higher filament loadings, e.g., 67.5-97.5% by volume (instead of 8.1-42.9% by volume). However, until the present invention, this has not been possible because the fiber or filament bundles separate during chopping and tumbling with the reduced volume fractions of resin. There is also a tendency for the resin to degrade if the temperature is raised to lower viscosity and enhance dispersion. Moreover, individual fibers can become airborne and cause problems in handling.
The improved elongated granule of the present invention solves such problems by substituting for the thermoplastic matrix separating and coating the fiber bundles, in the prior art, a much thinner layer of an efficient thermoplastic adhesive, which acts as a binder. As will be shown, such a judiciously selected binder will hold the fiber bundle together sufficiently to prevent broken bundles during chopping into elongated pellets and tumbling with the resin to be reinforced, and then the adhesive binder will readily break down in the presence of molten resin and thereafter not interfere with fiber dispersion, or degrade the resin properties, or constitute an environmental hazard.
As will be seen, the molding process itself can be used to disperse the resin uniformly throughout the molded part thus avoiding the compounding/pelletizing step.
As a decidedly unexpected advantage, and to further demonstrate the importance of the present invention, greater and more uniform dispersions of the fibers are achieved. It has been found that when using electrically conductive fibers, such as nickel coated graphite fibers, superior electromagnetic shielding is obtained at equal load levels (compared with compounded pellets), providing better shielding at one-half the cost, and, in comparison with the use of conductive, e.g., silver, paint there is much less or no secondary finishing with equivalent or better shielding, for superior physical properties, and superior long term reliability.